Self-purging centrifuge

ABSTRACT

A cylindrical drum is mounted for rotation about a vertical axis within a housing. A driven rotor is coaxially mounted within the drum. A fluid containing solid particles is introduced into the rotating rotor which throws the fluid toward the drum, causing rotation of the drum. As the drum rotates the particles collect on the drum side wall. An inwardly extending annular baffle prevents fluid from escaping from the bottom of the drum during normal operation, and clarified fluid is discharged through ports in the top cover of the drum. To purge the drum of accumulated particles, the drum is braked while the rotor continues to rotate. There is substantial clearance between the rotor and the drum. With the drum braked, the rotor creates a washing or eroding action in the fluid which breaks the particles free from the drum side wall, allowing them to be resuspended in the fluid. Fluid with resuspended particles flows over the baffle and out the bottom of the drum.

United States Patent Dudrey Jan. 21, 1975 SELF-PURGING CENTRIFUGEPrimary Examine'rGeorge H. Krizmanish 75 Inventor: Denis L'Dudrey,Minneapolis Attorney, Agent, or Firm-Merchant, Gould, Smith &

Minn.

[73] Assignee: Donaldson Company, Inc.,

7 Minneapolis, Minn. ABSTRACT [22] Filed: June 20, 1973. A cylindricaldrum is mounted for rotation about a vertical axis within a housing. Adriven rotor is coaxi- [21] Appl' 37l854 ally mounted within the drum. Afluid containing solid particles is introduced into the rotating rotorwhich [52] US. Cl 233/7, 233/27, 233/47 R, thrOWS the fluid toward thedrum, causing rotation of 233/23 R, 259/107 the drum. As the drumrotates the particles collect on [51] Int. Cl B04b 9/06 the drum sidewall. An inwardly extending annular [58] Field of Search 233/3, 7, 8, 1D, 14 R, baffle prevents fluid from escaping from the bottom of 233/14A, 19 R, 19 A, 21, 22, 23 R, 27, 28, the drum during normal operation,and clarified fluid 46, 47 R is discharged through ports in the topcover of the drum. To purge the drum of accumulated particles, the drumis braked while the rotor continues to rotate. [56] References CitedThere is substantial clearance between the rotor and UNITED STATESPATENTS the drum. Withthe drum braked, the rotor creates a 2,395,2862/1946 Merle 233 27 x Washing or emhhg action in the fluid which breaks2,436,493 2/194g strezynski H 233/21 x the particles free from the drumside wall, allowing 2,587,206 2/1952 Pattinson 233/21' them to beresuspended in the fluid. Fluid with resus- 3,424,375 1/1969 Maurer233/7 X pended particles flows over the baffle and out the bot-3,443,748 5/196 9 Hooper 233/8 X tom of the drum.

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Z l l .I 1- 'h -l 1 SELF-PURGING CENTRIFUGE BACKGROUND OF THE INVENTIONThe invention pertains to self-purging centrifuges and in particular toa centrifuge wherein the purging involves the setting up of a washing oreroding action.

A known way of purging a centrifuge involves scraping or plowing theaccumulated solid particles. This method is shown in US. Pat. No.3,403,848 (Windsor et al.'). In Windsor there is substantially noclearance between the rotor blades and the side wall of the bowl. Whenthe rotor is braked it merely scrapes the accumulated particles from theside of the bowl and they drop through an annular baffle which is raisedtoward an opening in the bottom of the bowl.

There are some problems associated with the Windsor apparatus. If fluidswith dense solid particles are being centrifuged the rotor may becomefirmly embedded in the accumulated particles so that when the rotor isbraked, the braking force is also transmitted to the bowl, and scrapingaction does not occur. Furthermore, apparatus using the scraping methodin purging must be designed to withstand relatively large'shockforces.In the present invention, the forces set up durwell as soft, solidparticles are completely purged.

SUMMARY OF THE INVENTION The invention is a self-purging centrifugeincluding a drum and a rotor assembly. The drum hasa side wall and a topwall member and is mounted, for rotation about a generally verticalaxis. The rotor assembly is coaxially mounted within the drum forindependent rotation. The rotor assembly includes a plurality of vanes,the outer edges of which are spaced a substantial distance from the sidewall of the drum..Contaminated fluid which is to be centrifuged isintroduced into the rotor assembly. The contaminated fluid containssolid particles which are to be separated from the fluid. A drivingmeans rotates the rotor assembly at a predetermined. rate. The rotatingrotor assembly throws the contaminated fluid outwardly toward the sidewall of the drum. The rotor assembly imparts a rotating movement to thecontaminated fluid directly toward the side wall of the drum, whichcauses the drum in turn to rotate. As contaminated fluid is introducedinto the rotor assembly and directed toward the side wall of the drum,an annular wall-of fluid is built up along the side wall of the drum.Centrifugal forces cause the particles to accumulate along the side wallof the drum. The inner portion of the wall of fluid is clarified becausethe solid particles are forced outwardly. The clarified fluid isdischarged through openings in a top wall member of the drum. Theopenings are spaced from the side wall of the drum and the blades of therotor assembly extend beyond the openings. An annular baffle extendsinwardly from the bottom of the side wall of the drum. The inner edge ofthe baffle is spaced from the side wall of the drum a greater distancethan the outermost area of the openings in the top wall member of thedrum so that liquid escapes out the openings rather than over the edgeofthe baffle. Particles are collected in the area on the side wall ofthe drum between the top wall member of the drum and the baffle on thebottom of the side wall of the drum. Braking means abruptly slows orstops the rotation of the drum to initiate a purge cycle.

In the purge cycle the rotating fluid wall is disrupted and theaccumulated-particles are resuspended in the fluid The purge cycle isended as the fluid with the particles resuspended drains over the baffleand out of the lower portion of the drum. I

The clarified fluid is collected in an annular chamber in a housingwhich surrounds the drum and the rotor assembly. The ring of openings inthe top wall member of the drum, through which the clarified fluid isdischarged from the drum, are covered with a ring having louvers formedtherein. The louvered ring prevents the discharge of fluid from the drumduring the purge cy cle. The louvered ring is mounted over the inside ofthe openings. A shield in the form ofa ring is mounted adjacent theoutside of the openings to prevent splashing. The annular baffle is arim-like member and can be removed from the side wall of the drum andreplaced BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational viewof the centrifuge, portions thereof being broken away and shown insection;

FIG. 2 is a sectional view of FIG. 1 as taken generally from theirregular line 22 of FIG. 1;

FIG. 3 is a sectional view as seen from the line 33 of FIG. 1 showingthe bottom area of-the drum from which the purged fluid is discharged;and

FIG. 4 is a sectional view as seen from the line 4-4 of FIG. 1 showingthe rotor assembly vanes and the top wall member of the drum.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, thecentrifuge has a drum including a cylindrical side wall 10, a top wallmember 12 and a frustoconical bottom member 14. The periphery of wall 14is attached to the bottom edge of side wall 10 and the center extendsinto the drum. The rotor assem- 'bly includes a shaft 16, a fin assembly18 bolted to shaft 16, and a set of flat, plate-like vanes 20 bolted tothe fin assembly 18. In the embodiment shown, vane set 20 includes fourvanes spaced degrees apart; The drum and the rotor assembly are mountedin a housing including a generally cylindrical body '22, a top cover 24,and a bottom p ortion'26. The drum and the rotor'assembly areconcentrically mounted and rotate independently about a vertical axis.The drum rotates on bearings 28 and 30 mounted in cover 24 and bottommember 14, respectively, and the rotor assembly rotates in bearings 32and 34 mounted in wall member 12 and bottom portion 26, respectivelyfAmotor (not shown) drives the rotor assembly through a set of belts 36and a pulley 38 mounted on shaft 16.

Contaminated fluid, that is, fluid containing solid particles, entersthe centrifuge through a feed pipe 40 and is introduced into therotating assembly. The contaminated fluid, which for example may bemachine tool coolant carrying metal chips or the like, is thrown ordirected outwardly by the rotating fln assembly 18 and vanes 20 towardside wall of the drum. The drum is driven by the viscous or shear forcesassociated with the fluid that is directed outwardly and rotated by thefin assembly 18 and vanes 20.In steady state operation the rotationalrate of the drum lags behind that of the rotor assembly by 50 to 300rpm. Typically the rotor assembly is driven at about 3,000 rpm. As thedrum and rotor assembly rotate a wall of fluid is built up on side wall10 of the drum. Centrifugal forces cause the particles in the fluid tobe thrown radially outwardly to accumulate in the portion of the fluidwall along side wall 10 of the drum.

As the wall of fluid builds up, clarified fluid is discharged through aset of equally spaced openings or ports 42 formed in a circle concentricwith shaft 16 in top cover 12 of the drum, and is collected in anannular chamber 44 within the housing which surrounds the drum. Theouter wall of chamber 44 is formed by body 22 of the housing and theinner and bottom walls are formed by wall members 46 and 48,respectively. A ring 50 having louvered openings therein is placed overthe inside of ports 42 to prevent escape of contaminated fluid throughthe openings 42 during the purge cycle. A ring-like baffle or shield 52is placed over the outside of the openings 42 to prevent splashing asthe clarified fluid is being discharged. Baffle 52 is spaced above theopenings 42 and is secured to top wall member 12 at its inner edge toprovide an unobstructed discharge space facing outwardly toward a curvedsplash cone 54 which directs the outwardly thrown fluid downwardly intochamber 44.

An air brake 56 is used to slow and stop the drum when the purge cycleis initiated. When brake 56 is actuated, a shoe 58 is driven upward andheld against a projection 60. Projection 60 is in effect an extension ofthe drum.

An annular rim or baffle 62 having a generally triangular cross sectionis secured to the bottom of the drum side wall 10 with one side wallthereof attached to the drum side wall and an opposite apex thereofforming a circular edge or lip 64 over which the fluid and resuspendedparticles flow during the purge cycle. The inner edge or lip 64 ofbaffle 62 extends inwardly beyond the centers of ports 42. Note alsothat the outer edges of vanes extend beyond the outermost edges of ports42. Particles are accumulated on the side wall 10 between the uppercover 12 and the baffle 62.

When the drum is stopped or slowed the rotating vanes 20 disrupt thefluid, and the accumulated particles are resuspended in the fluid. Asthe purge cycle continues, the fluid and the resuspended particles flowinwardly and downwardly along the upper surface 72 of rim or baffle 62,downwardly over lip 64, outwardly through openings in side wall 10,downwardly through a series of circumferentially spaced openings 27 inbottom portion 26 and finally through an outlet 66 formed in a lowerwall 23 of housing 22.

Referring to FIG. 3 the posts are referenced with numeral 68. Posts 68are formed such that the sides of the posts are substantially alignedwith the flow of the fluid as it is discharged from the bottom of thedrum during the purge cycle. Thus posts 68 do not impede the outwardflow which circulates around the inner surface of body 22. This preventsthe fluid that is discharged during purging from reaching bearings 30and 34.

An access opening 69 is formed in side wall 22 radially outwardly of thedischarge openings of drum 10. The opening 69 provides a convenientmeans to gain access to screws which mount the baffle 62. Thus, thebaffle 62 may be removed and replaced without complete disassembly ofthe centrifuge. The opening 69 also forms a convenient means to clear ordislodge solids which may accumulate in the area adjacent the base ofthe rotor assembly.

As shown in FIG. 4 the louvered ring 50 prevents the flow of fluidthrough ports 42 during the purge cycle. Assuming that the rotorassembly and the fluid is moving to the right the fluid would have todouble back to the left before it could flow up and out ports 42. I havefound that this does not occur.

As shown in FIG. 2, clarified fluid collected in chamber 44 is removedfrom the centrifuge through an outlet pipe 70.

The purging of solids is accomplished partly by utilizing theinstability of the boundary layer created at the inner surface of sidewall 10 when the drum is slowed or stopped by the brake. A rotating flowis unstable if the magnitude of VR decreases with R, where VR equals theproduct of tangential velocity (V) and radius (R).

Where VR decreases with increasing R the centrifugal force acting uponan increment of fluid near the axis of rotation is greater than theforce acting on an increment of fluid at a larger radius. Therefore,fluid near the axis tends to move outwardly toward the drum side wall 10to a greater extent than the fluid that is near side wall 10. As theinner fluid moves outwardly the outer fluid' near the side wall I0 isdisplaced.

The conditions for unstable rotating flow always exist at the boundarylayer between a rotating flow (i.e., body of fluid) and the wall ofastatic container (braked drum); Fluid in the boundary layer, slowed bythe frictional drag of the drum wall, tends to be displaced radiallyinward by the faster moving body of fluid nearer the axis of rotation.This interchange of fluid is greatly aided by the presence of a radialsurface since the boundary layer at the radial surface is also unstableand provides a path for fluid at the wall to travel inward. This radialsurface is shown as downwardly and inwardly sloping surface 72 of baffle62 in FIG. 1. In the preferred embodiment, surface 72 is slanteddownward but this is not necessary and surface 72 may be relativelylevel, i.e., horizontal.

In the purge cycle the drum is stopped, but the rotor assembly continuesto be driven. The increased turbulence set up in the fluid near vanes 20resuspends the particles accumulated along the wall and promotesdrainage by a secondary flow phenomenon.

Although not necessary, the incoming flow of contaminated fluid isgenerally shut off or reduced during the purge cycle. The purge cyclemay be initiated either at periodic intervals or by a sensor. In anycase purging takes place before the accumulated particles reach thevanes 20 so that a significant quantity of fluid remains in the drum forerosion and resuspension of the particles, and the vanes are notembedded in the accumulated particles thus restricting or preventing therelative motion of the drum and rotor assembly which is necessary forproper purging.

As drum rotation is stopped, two separate equally im' portant actions ofthe purge cycle begin. The first is the washing or erosion andresuspension of accumulated particles. This is accomplished by thewashing action caused by the rotational speed of the rotor assemblywithin the drum as the drum is being braked. Once the particles areloosened they are resuspended in the rapidly moving fluid.

The second action, occuring simultaneously with the first, is thedrainage of the particle-laden fluid due to the secondary flow patterncreated by the braking of the drum. A slow moving boundary layer iscreated adjacent side wall 10. The faster moving fluid of the innerportion of the wall of fluid tends to displace the boundary layer fluid.The radial surface 72 provided by the baffle 62 at the bottom of thedrum provides a means for the exchange of fluid to take place. Slowmoving boundary layer fluid flows down and inward along surface 72. Thefaster moving fluid, displacing the boundary layer fluid, is itselfslowed by side wall 10. The profile of the wall of fluid within the drumbecomes thicker at the bottom because of the downward boundary layerflow, and becomes thinner opposite the rotating vanes which maintainhigh rotation of the fluid. A circulatory flow pattern is set up, withslow moving fluid from the thick lower portion experiencing a lowcentrifugal force, flowing upward along the inner surface of the fluidwall to take the place of the faster, outwardly moving fluid,experiencing high centrifugal forces, which has displaced the boundarylayer fluid. The opening in the bottom of the drum formed by baffle 62is sized to allow a'controlled rate of fluid flow out of the drum withthis circulatory flow pattern as described. As some fluid flows out, theeffect of the vanes decreases, allowing fluid to flow out faster untilall of the fluid slides out the bottom opening.

The above discussion of the action of the fluid wall at the bottom ofthe drum is also true to a lesser extent at the top of the drum wherethedrum cover 12 provides aradial surface for the boundary flow. Thephenomenon at the top is not as effective because of the close proximityof vanes 20. When thickening of the fluid wall occurs at drum cover 12,fluid extends inside of ports 42 and contaminated fluid can over flowinto the clean section of thecentrifuge. To prevent this, louvers 50 areplaced over each of the ports 42. The louver openings are aimed so that,with the drum stopped, the fluid, which is rotating at a high velocity,must double back in order to flow out through ports 42. Therefore at thehigh relative speed involved during purge, fluid is unable to exitthrough the ports 42. During normal operation, the speed of the rotor,drum and fluid is about the same so as not to interfere with thedischarge of clarified fluid through ports 42. As shown, the louvers 50are formed as part of an annular ring. However, each louver 50 may beformed from a separate piece of material and mounted to the inside oftop cover 12.

An important variable effecting the purging of solids is the fluiddischarge time, that is, the time it takes for the drum to be emptied offluid when the brake is actuated and the input flow-of contaminatedfluid is shut off. Fluid discharge time can vary from three or fourseconds upward and is influenced most by the inner diameter of baffle 62and the viscosity of the fluid. The boundary layer created when the drumis stopped causes a thickening of the fluid wall at the bottom of thedrum. 1

If the inner diameter of baffle 62 is too small, fluid will not drainfrom the drum but will continue to recirculate within the drum under theinfluence of the vanes 20. lfthe thickening portion of the fluid extendsinward of the edge 64 of baffle 62, fluid will begin to drain from thedrum at a rate determined by the size of the drum opening and the fluidprofile.

For a given rotor assembly speed and centrifuge design, the fluidprofile is determined most by viscosity and to a'lesser extent by thedepth that vanes 20 extend into the wall of fluid. During the dischargeperiod of the purge cycle, previously accumulated particles are beingeroded by the shear forces imposed by the rapidly rotating fluid. If thedischarge period is very brief, for example three to five seconds, notall of the caked particles will be resuspended and therefore theparticles will not be completely purged from the drum. Obviously iflittle or no liquid is discharged, that is if the discharge period isinfinite, particles will not be purged even though suspended.

It has been found that a discharge time between 10 and 15 seconds is themost practical. The inner diameter of baffle 62 becomes critical abovethis range so that fluid may not discharge at all because of slightdecreases in viscosity due to temperature rise. For a purge time of lessthan about 10 seconds, particlesmay not be completely resuspended.Dependent upon the range of fluid, viscosities that are encountered bythe selfpurging centrifuge, the inner diameter of baffle 62 may need tobe sized for the particular application. Two or three baffles withdifferent inner diameters may be sufficient to cover a range. Someextension of the effective discharge time is possible by permittinginput flow of contaminated fluid to continue for a few seconds after thedrum has been stopped. The depth the vanes pene- 'trate into the wall offluid has an additional effect on discharge time and erosion forces.However, benefits gained from increasing the depth ofvanes 20 into thefluid beyond about onequarter inch do not appear commensurate with thegreater forces and loads on the drive motor and belt, etc. Adjustablevanes may be appropriate and at any rate the vanes must be replacedperiodically because they are subject to erosion durin the purge cycle.

The quantity of particles affects the ability to purge in two ways.First, with a large accumulation of particles there are more particlesto be erode'dand suspended. Secondly, as the quantity of particlesincreases, the quantity of fluid available for erosion and resuspensionof the particles decreases.'ln tests of a model of the embodiment of theinvention described above, the limit of what can be purged effectivelyappear to be about cubic inches of particles (approximately one-third ofthe fluid volume) for the contaminants'tested. This limit is variableand depends upon the type of contaminant. In the embodiment tested thefluid capacity of the drum is about 5 quarts.

Ideally the purge cycle is initiated whenever the accumulation ofparticles reaches the maximum amount that can be reliably purged. Thisresults'in the least amount of fluid passing through the drag-out orsettling tank, the least wear and tear on the apparatus, and the leastdown-time for the centrifuge, i.e., time when it is not centrifuging andclarifying fluid.

The time it takes to accumulate the proper amount of particles dependsonthe generation rate. In many applications where there is not anextreme variation in particle generation rate, the purge cycle can beinitiated on a timed-interval basisv This interval can be set based onan estimate of the particle generation rate of the system and thenadjusted through experience. This kind of control can be accomplished bya reset timer (adjustable inv the range of minutes to several hours)activating a program timer which controls the sequence of events duringa purge cycle of about to 30 seconds.

The sequence of events during the purge cycle can vary. The basicpattern is for the brake to be activated and the feed pump to be shutoff at the beginning of the purge cycle and for the brake to be releasedand the pump turned on again after a delay of about 15 seconds. Thismethod is satisfactory in most instances, In some cases, however, wherethe particles are difficult to purge, the sequence of events can bechanged. For example the feed pump can be allowed to run for a fewseconds after the drum has been braked. This extends the time duringwhich particles are being eroded or washed and provides additional fluidfor resuspension ofthe eroded particles. Alternatively the fluid flowduring the purge cycle may be reduced rather than shut off completely.In this way the purge cycle can be extended indefinitely.

What is claimed is:

. A self-purging centrifuge, comprising:

a. a drum having a side wall and a top wall member,

mounted for rotation about a generally vertical b. a rotor assemblycoaxially mounted in said drum for independent rotation with respect tosaid drum, said rotor assembly comprising a plurality of vanes havingthe outer edges thereof spaced a substantial distance from said sidewall of said drum;

c. means for introducing a fluid containing solid particles to beseparated therefrom, into said rotor assembly;

d. means for driving said rotor assembly at a predetermined rotationalspeed to throw the fluid and solid particles outwardly toward said drumside wall, said fluid rotating therewith, and said rotating fluid inturn causing rotation of said drum;

. said top wall having openings therein for discharging clarified fluid,said openings each having an outer edge spaced from said side wall;

. an annular baffle extending inwardly from the bot- 2. The apparatus ofclaim 1 wherein said openings in said top wall are arranged in a circleformed about said axis.

3. The apparatus of claim 2 wherein said rotor assembly comprises aplurality of radially extending flat vanes each having a vertical outeredge thereof terminating between said openings in said top wall and saidside wall.

4. The apparatus of claim 3 wherein said outer edges of said vanes arepositioned closer to said openings than to said side wall to therebyprovide an open annular collection area of substantial depth betweensaid vanes and said side wall.

5. The apparatus of claim 1 including louver means mounted inside saidtop wall member and over said openings, said louver means having louveropenings aimed in the direction of fluid rotation, said louver meansthereby blocking passage of fluid and resuspended particles through theopenings during the purge cycle while permitting flow through saidlouver openings during normal operation.

6. The apparatus of claim 1 wherein said baffle is a rim-like memberremovably secured to said drum side wall.

7. A self-purging centrifuge, comprising:

a. a housing having a vertical axis;

b. a drum having a cylindrical side wall, and top and bottom wallmembers, mounted for rotation in said housing about said axis;

c. a rotor assembly coaxially mounted in said drum for independentrotation with respect to said drum, said rotor assembly comprising aplurality of radially extending flat vanes each having a vertical outeredge thereof spaced a substantial distance from said side wall of saiddrum to define a particle collection area therebetween;

(1. means for introducing a fluid containing solid particles to beseparated therefrom, into said rotor assembly;

e. means for driving said rotor assembly at a predetermined rotationalspeed to throw the fluid and solid particles outwardly toward said drumside wall, said fluid rotating therewith, and said rotating fluid inturn causing rotation of said drum;

f. said top wall having openings therein, spaced from said side wall,for discharging clarified fluid, said drum side wall collecting thesolid particles being centrifugally removed therefrom;

g. an annular rim extending inwardly from said drum side wall andadjacent said bottom wall, to further define with said top walls saidparticle collection area therebetween, said rim having an inner lipspaced from said side wall a greater distance than said openings in saidtop wall;

h. means for abruptly slowing the rotation of said drum to initiate apurge cycle by disrupting the to tating fluid and thereby resuspendingthe separated particles in the fluid; and

i. drain means in said drum below said rim for draining the fluid andresuspended particles from said drum during the purge cycle.

8. The apparatus of claim 7 including louver means mounted inside saidtop wall member and over said openings, said louver means having louveropenings aimed in the direction of fluid rotation, said louver meansthereby blocking passage of fluid and resuspended particles through theopenings during the purge cycle while permitting flow through saidlouver openings during normal operation.

9. A self-purging centrifuge for clarifying contaminated fluid,comprising:

.a. housing;

b. a drum rotatably mounted within the housing, the

drum having top and side walls, the top wall containing ports for thedischarge of clarified fluid, the drum containing a bottom opening fordraining purged fluid;

. a rotor assembly rotatably mounted in the drum for directingcontaminated fluid toward the side wall of the drum, said drum side wallcollecting a layer of solid particles being centrifuged from the fluid,the rotor assembly including vanes, the outer edges of which extendoutward beyond the discharge ports and are located a substantialdistance from the side wall so as not to become embedded in the layer ofparticles, the outer portions of the vanes and the side wall of the drumgenerally defining a g. means for collecting the clarified fluid;

h. means for braking one of the drumand rotor assembly; and i. louvermeans for deterring the discharge of fluid through the discharge portswhen braking occurs.

1. A self-purging centrifuge, comprising: a. a drum having a side walland a top wall member, mounted for rotation about a generally verticalaxis; b. a rotor assembly coaxially mounted in said drum for independentrotation with respect to said drum, said rotor assembly comprising aplurality of vanes havinG the outer edges thereof spaced a substantialdistance from said side wall of said drum; c. means for introducing afluid containing solid particles to be separated therefrom, into saidrotor assembly; d. means for driving said rotor assembly at apredetermined rotational speed to throw the fluid and solid particlesoutwardly toward said drum side wall, said fluid rotating therewith, andsaid rotating fluid in turn causing rotation of said drum; e. said topwall having openings therein for discharging clarified fluid, saidopenings each having an outer edge spaced from said side wall; f. anannular baffle extending inwardly from the bottom of said drum sidewall, to define with said top wall, said side wall and said outer edgesof said vanes a particle collection area therebetween, said bafflehaving an inner edge spaced from said side wall a greater distance thanthe outermost edges of said openings in said top wall; and g. means forabruptly slowing the rotation of said drum, to initiate a purge cycle bydisrupting the rotating fluid and thereby resuspending the separatedparticles in the fluid, said liquid and resuspended particles drainingfrom said drum over said baffle during the purge cycle.
 2. The apparatusof claim 1 wherein said openings in said top wall are arranged in acircle formed about said axis.
 3. The apparatus of claim 2 wherein saidrotor assembly comprises a plurality of radially extending flat vaneseach having a vertical outer edge thereof terminating between saidopenings in said top wall and said side wall.
 4. The apparatus of claim3 wherein said outer edges of said vanes are positioned closer to saidopenings than to said side wall to thereby provide an open annularcollection area of substantial depth between said vanes and said sidewall.
 5. The apparatus of claim 1 including louver means mounted insidesaid top wall member and over said openings, said louver means havinglouver openings aimed in the direction of fluid rotation, said louvermeans thereby blocking passage of fluid and resuspended particlesthrough the openings during the purge cycle while permitting flowthrough said louver openings during normal operation.
 6. The apparatusof claim 1 wherein said baffle is a rim-like member removably secured tosaid drum side wall.
 7. A self-purging centrifuge, comprising: a. ahousing having a vertical axis; b. a drum having a cylindrical sidewall, and top and bottom wall members, mounted for rotation in saidhousing about said axis; c. a rotor assembly coaxially mounted in saiddrum for independent rotation with respect to said drum, said rotorassembly comprising a plurality of radially extending flat vanes eachhaving a vertical outer edge thereof spaced a substantial distance fromsaid side wall of said drum to define a particle collection areatherebetween; d. means for introducing a fluid containing solidparticles to be separated therefrom, into said rotor assembly; e. meansfor driving said rotor assembly at a predetermined rotational speed tothrow the fluid and solid particles outwardly toward said drum sidewall, said fluid rotating therewith, and said rotating fluid in turncausing rotation of said drum; f. said top wall having openings therein,spaced from said side wall, for discharging clarified fluid, said drumside wall collecting the solid particles being centrifugally removedtherefrom; g. an annular rim extending inwardly from said drum side walland adjacent said bottom wall, to further define with said top wallssaid particle collection area therebetween, said rim having an inner lipspaced from said side wall a greater distance than said openings in saidtop wall; h. means for abruptly slowing the rotation of said drum toinitiate a purge cycle by disrupting the rotating fluid and therebyresuspending the separated particles in the fluid; and i. drain means insaid drum below said rim for draining the fluid and reSuspendedparticles from said drum during the purge cycle.
 8. The apparatus ofclaim 7 including louver means mounted inside said top wall member andover said openings, said louver means having louver openings aimed inthe direction of fluid rotation, said louver means thereby blockingpassage of fluid and resuspended particles through the openings duringthe purge cycle while permitting flow through said louver openingsduring normal operation.
 9. A self-purging centrifuge for clarifyingcontaminated fluid, comprising: a. housing; b. a drum rotatably mountedwithin the housing, the drum having top and side walls, the top wallcontaining ports for the discharge of clarified fluid, the drumcontaining a bottom opening for draining purged fluid; c. a rotorassembly rotatably mounted in the drum for directing contaminated fluidtoward the side wall of the drum, said drum side wall collecting a layerof solid particles being centrifuged from the fluid, the rotor assemblyincluding vanes, the outer edges of which extend outward beyond thedischarge ports and are located a substantial distance from the sidewall so as not to become embedded in the layer of particles, the outerportions of the vanes and the side wall of the drum generally defining avolume within which the contaminated fluid is centrifuged; d. means fordriving the rotor assembly; e. means for introducing contaminated fluidinto the rotor assembly; f. a rim mounted on the side wall of the drumat the lower end of the centrifuging volume, the rim extending inwardbeyond at least the outer edges of the discharge ports and providing aradial surface; g. means for collecting the clarified fluid; h. meansfor braking one of the drum and rotor assembly; and i. louver means fordeterring the discharge of fluid through the discharge ports whenbraking occurs.